Worm gear drive



8- 1952 J. R. SHONNARD ETAL 2,608,103

WORM GEAR DRIVE Filed Jan. 28, 1949 ECCENTRIO INVENTORS JOHN R. SHONNARD HANS F. STEEN BY 2 Q Fm E Patented Aug. 26, i952 WORM GEAR DRIVE John R. Shonnard, New York, and'Hans F. Steen, Jamaica, N. Y., assignors to Times Facsimile Corporation, New York, N. Y., a corporation of New York Application January 28, 1949, Serial No. 73,27 6

This invention relates to power drive gearing and more particularly, to'worm'gearing for facsimile machines, sound-recorders and reproducers having a turntable, and other machines Wheresmooth; constant-speed rotation ofa driven member is essential.

Straightfworm gearin has been commonly employed for obtaining large speed reduction between two shafts. The section of the worm thread is straight-sided, as in the case of an involute rack tooth. Hence the worm may be readily cutor hobbedwith the required accuracy to obtain reasonably good efiiciency. The force transmitted by the gearing produces end thrust on the worm and gear shafts, and a bending force radial to the worm tending to push the worm and worm gear apart. Excessive wear is usually caused by failure of lubrication resulting from high intensity of pressure between the teeth on the worm and gear sufficient to squeeze lubricant from the contacting-surfaces, and this is aggravated'by any irregularity in drive ratio because of the resulting contact pressure and displacement'of the teeth by the torsional-and bending forces.

"Straight worm gearing, as used heretofore, has

an" inherent drive-ratio error, producing a pulsatio'n" or speed fluctuation in the driven member. In many cases, this speed variation is extremely-"undesirable, and torsional decouplers and fly-wheels; have been used to'reduce-the angular-speed fluctuations of the driven member.

Such arrangements are complicated and only partially eifective but in spite of these drawbacks are used to a great extent because of the importance of reducing the angular velocity variations as much as possible in certain machines; In general terms, the object of the present invention is to overcome these drawbacks and to provide worm gearing in which the change in drive ratio is minimized or completely eliminated ina s'impleand effective manner. 7

A further object of the invention is to provide'an improved worm drive which has an inherently constant drive ratio, thus eliminating speed fluctuations 3 without auxiliary stabilizing means,- and decreasing wear. I

-A still'further object of the invention is to provide improved means for imparting predetermined eccentric, orbital movement of the worm about its-longitudinal rotative axis, preferably adjustableduring assembly of the gear drive to obtain the desired corrective efiect.

Other objects and advantages of the inventionthe worm, torsion and consequent bending of a 9 ,Claims. (01. 74-396) will become clear from the following description: of the preferred embodiment thereof show'n in" the accompanying drawings, wherein Fig. 1 is an explanatory diagram of the the worm and worm gear are fixed;

Fig. 2 is a sectional'view of thedrive shaft and worm in a driveembodyingthe invention; 5

Fig. 3 is a detail view of the slotted end of the F 'worm;.and Fig. 4. is an assembly view of a typicalworm drive according to the invention.

Referring to Fig. 1, the contacting teeth of the worm and worm gear of a straight worm drive are indicated at It) and II, respectively. As the driving worm rotates, the thread and'gear toothmay be assumed to advance from the positions" shown in full lines tothat shown indottedlines', if the rotational axes of the Worm and geararefixed. It will be noted that, as the -=worni rotates, the point of contact betweenthe teeth changes from a to I). Since the point 17 is nearer to the axis of the Worm gear, as represented-by the distance 0, and the profile of the tooth o'r thread I0 slopes at an angle to the radius of the-- worm gear extendingto the contact point, it will" be evident'that the drive ratio betweenthe worm- As the rotation cos- I tinues, before the tooth on the wormgear leaves the worm thread, the movement-of the contact point reverses, thus caus'ingan opposite change; j indrive ratio. Therefore if'the wormis rotating atauniform speed, a'pulsation in speed isi'mparted to the worm gear as each tooth-engagesand worm gear changes.

the worm thread. In accordance withtheinvention, this driving pulsation orangula yvelo city error ofthe (driven) worm gear is minimized'by imparting a' predetermined orbital movement' t'o f the worm about its own axis. While the usual construction, the worm is the driving element} obviously the invention may also be employed;

where it is the driven element. v

.Referring to Figs. 2, 3 and 4, a typical worm gear reduction drive is illustrated Fig. 4. The T shaft of a motor 14 is shown as carrying-or at tached to a worm I5 engaging aworm' gear" [6 constituting the driven member. Instead of supporting the worm I'5 for. rotation on a fixedaxis as in ordinary worm gearing, in accordance i with the invention and as shown in detail in Figs.

2 and"3,'.the worm is eccentrically mounted onthe motor shaft l8.

The shaft l8.is intended to opera' tion of straight worm gearingwhere the axes of represent any-drW-- mg member or'shaft supported for rotation about" a fixed axisfas for example by-"the bearings l9" and 20 and an axial thrust bearing 2|. While the drive shaft l8 rotates about a fixed axis, the worm 15 as previously stated, is supported for eccentric or orbital movement about the axis of said shaft. As shown by way of example, the worm I is provided with an eccentric extension or shank 22, parallel to the axis of thegworm, which is: mounted orfitted in an eccentric bore 23 in the end of the drive shaft l8. Preferably the eccentricity of the extension 22 and of the bore 23 may be substantially equal so that by rotating the shank in the bore, the eccentricity of T the worm IS with respect to the axis of the shaft It may be varied from substantiallyzero to a maximum value.

It will be understood that a predetermined amount of eccentricity as well as a definiteaxial position of the worm is required for optimum correction of the drive ratio error produced as described above in connection with the diagram inilg; 1. Tothisend the worm may bevadjust able in -position.- As shown it is hollow and the extension 22. slotted so that it can be wedged into tight clamping contact with the bore on the shaft [8 =by;meansof a tapered pin 25-, the. upper end 26 of which isthreaded for engagement with the threaded-portion 21 of the'worm I5. A'socket'nis also provided in the head of the taper-pin 25 for'an Allen-head or hexagonalsocket wrench. During assembly, the'worm I5 is locked in position on the drive shaft I8 in various axial and angular positions by tighteningg-the taper pin 25 until correction is obtained for the angular velocity error of the wormdrive.

It is found that by using. suitable test or in-' dicating equipment in connection with the driven member or gear IS, the angular velocity error of the-worm drive may be quickly eliminated if the worm-is adjusted in-position by the trial and error method.

Since the construction described has proven effective to5completely eliminate the :spa'ced pulsations or variations'vof the driven gear, it'is probable that the adjustment of the position of the'worm is. effective to take care of 7 any errors in the rotational. mountings for the worm and worm gearflsuch as variations in spacing; between the centers of the shafts, as well-as the; inherent variations in the contact surfaces of the teethas indicatedin Fig. 1. Irrespective of the theory of operation of the improved construction, it hascbeen'found in actual practice that the angular velocity errorof worm gearing'of' the character described can be corrected in a simple and-' eifectivemanner. It appears to'be necessary. to-provide for an eccentricity of the'worm usually of less than about of the pitch diameter of the worm-itself, although theinvention is nottobe limited to any specific dimensions or proportions. Asanrexample of a workable worm'andgear,.in accordance with the invention, a single-thread worm having an outside diameter of 0.31 inch, 13threads'per inch and a pitch diameter 'of 0.267 inch; for use with'a 20- tooth worm gear having. a pitch diameter of 0.5. inch, was-cut withan eccentric shank 0.002 inch'off center, thus providing a maximum eccentricity'of 0.004-inch with respect tothe axis ofther'motor shaft I8.

It will be seen that theinvention provides-an improved wormi drive which is suitable for facsimile machines, sound reproducers: and other machines in: which even slight speed fluctuations of the drivenzmember are-undesirable; When the worm is properly-mounted,.a uniform drivev ratio is obtained through the corrected driving engagement between the worm threads and gear teeth, thus reducing wear and vibration to a minimum. Since the worms and drive shafts may be made alike in quantity production, within the limits of ordinary shop tolerances, and any variations in the-fit of the parts! taken care of by the adgiustablamounting ofthe worm, high precision workmanship is unnecessary. In addition, when a worm is worn out or damaged, it may be readily replaced without affecting the dsiredwharacteristics of the worm drive.

While a preferred embodiment of the invention has-been described in detail in order to explain the underlying principles, various modifications in the construction and arrangement of parts will occur to those skilled in the art and may be made withoutdep'arting from the scope of the invention.

We claim:

1. In worm gearing. having uniform drive ratio, a. single-thread. worm meshing. with... the worm gear, -a-support ing..shaft for said worm rotatably supported in fixed bearings, said wotmt being eccentric with respect to. the axisuolfisaid.v shaft and means for securingsaid: worm-on supporting shaft with the. amount -ofeccentricity required for uniformudrive ratio inall angular positions of the worm;

2. In gearing of. the class described, in com-.- bination, a worm having astraight sidedthreadr, a: worm gear meshing with said. worm, meana.

for adjusting the-position of said. wormdn an.

axial direction and meanstfor imparting-z ap e determined orbital.v movement of the worm v around its longitudinal axisiofrotation to-main.-' tain the drive ratio of the :gear constant.

3. In gearing of the class described, in com.- bination, a worm gear, arshaft-mounted for rota, tion on a fixed axis, a tworm-von saidshaftmeah ing-with-said worm gear, said shaftwandworm having bore and shank. portions respectively .of the. same diameter for mounting the. worm. on. said-shaft, said bore andashank portions being. eccentric with respecttosthe. axes of the. worm and shaft, and means for adjustably locking; the bore and Y shankportions. of the worm-and shaft together to provide desired eccentricity;- of the worm with respectto the fixed. axis.of shaft.

4. In gearing of the class .described,.in.combination, a worm gear, arotatable worm meshingwith said. worm gear, a shaftfor. supporting, said worm, saidworm being-eccentric. with..respect to the axis oftsaid. shaft, said-shafLand. worm having bore and shank portions ..ofl the. same diameter. for mounting the-wormon the shaft, one of said. portions being. located ,.to .pro vide desired eccentricity of the. worm andmeans: for securingthe. worm on. said shaft.

5. In combination,.a wormvgear, .a.rotatahle worm meshing with said gear, means including: an eccentric mounting, for: saidtworm .tQJmpart'. adjustable orbital movement to.the .wormtabout itsaxisofrotation-and-frictional-meansionlock ing; said eccentric mounting in position to pita vide desired eccentrieitytof" the /worm.

6. In combination, aworm-gear, aiworm mesh-- ing with. said gear, said. worm being-provided with an eccentric slottedeextensionhavinggan axial tapered -bore, a: shaftbored to. receiver-said. extensionof the worm-and atta-peredpin insaidtaperedv bore of the worm for. spreadingarthe: slotted extension to: clamp: the-wormtmsald: shaft. I 1

7. The combination recited in claim 6 in which the bore in said shaft is eccentric with respect to the axis of the shaft.

8. In combination, a worm gear, a rotatable worm, a rotatable shaft for said worm and frictional clamping means for securing said worm to said shaft with predetermined eccentricity between the axes of the worm and its shaft to correct the driving engagement of the worm with said worm gear.

9. In combination, a worm gear, a worm engaging said gear and a rotatable shaft for supporting said worm, said worm gear and shaft having fixed axes and said worm being adjustable in an axial direction with its central axis eccentric to the axis of the shaft, to correct the driving engagement between said worm and worm gear.

JOHN R. SHONNARD. HANS F. STEEN.

REFERENCES CITED The following references are of record in the file of this patent: V

UNITED STATES PATENTS Number 

